What is the general procedure to create an AI system that can detect fire in images?
Many AI libraries allow the ability to feed them an Image, but others have to be modified to allow this.

Disregarding the information above, as it is just a notification, you can do this in many ways. Depending on the library you use, different methods give varying accuracy results. 

The real questions are:


How complex do you want the NN to be?
Is this going to be commercial, or private?


For any of the answers above, you will have to find the best method that works for your needs. The more this matters to the world, the more complex it will have to be.